There has been known an optical information recording medium called as a DVD or a BD (a Blu-ray disc) among the commercially available high-density and large-capacity optical information recording media. In recent years, the optical information recording medium has been rapidly spread as a recording medium for recording images, music, and computer-processed data.
As the capacity of an optical information recording medium has been increased, a short-wavelength light source and an objective lens having a large NA to be used in an optical head device have been developed. As the NA is increased, however, a change in a spherical aberration resulting from a change in thickness of a light transmissive layer of an optical information recording medium is increased. For instance, in the case where a light source for emitting light of 650 nm wavelength, and an objective lens having an NA of 0.6 are used in information recording and/or reproducing with respect to a DVD, a spherical aberration of about 10 mλ is generated with respect to a change in light transmissive layer thickness of 10 μm. In the case where a light source for emitting light of 400 nm wavelength, and an objective lens having an NA of 0.85 are used in information recording and/or reproducing with respect to a BD as a next-generation optical information recording medium, a spherical aberration of about 100 mλ is generated with respect to a change in light transmissive layer thickness of 10 μm. Thus, a spherical aberration of about ten times of a spherical aberration in a DVD is generated in a BD. In view of this, a measure for correcting spherical aberration is necessary in the aforementioned optical head device.
For instance, patent literature 1 proposes an approach, wherein a collimator lens disposed between a light source and an objective lens is mounted on a collimator lens actuator, and the collimator lens is moved in such a direction as to cancel a spherical aberration resulting from a thickness error of a light transmissive layer. The approach is described in detail referring to FIG. 8.
FIG. 8 is a schematic construction diagram of a conventional optical head device. Referring to FIG. 8, the optical head device 120 includes a light source 101, a beam splitter 102, a quarter wavelength plate 103, a collimator lens 104, an objective lens 106, a multi lens 107, a photodetector 108, a two-axis actuator 109 for driving the objective lens 106, and a collimator lens actuator for driving the collimator lens 104.
Laser light emitted from the light source 101 is transmitted through the beam splitter 102, and incident into the collimator lens 104. The laser light incident into the collimator lens 104 is collimated into parallel light by the collimator lens 104, if the thickness of a light transmissive layer 131 of an optical information recording medium 130 is set to a defined value. The collimator lens 104 is mounted on the collimator lens actuator 110, and is movable in forward or backward direction along an optical axis of laser light by the collimator lens actuator 110.
The laser light transmitted through the collimator lens 104 is turned into circularly polarized light while being transmitted through the quarter wavelength plate 103, and incident into the objective lens 106. The laser light collected by the objective lens 106 and incident into an information recording surface of the optical information recording medium 130 is turned into return light by reflection on the information recording surface. After the return light is transmitted through the objective lens 106 along an incoming optical path, the return light is incident into the quarter wavelength plate 103. The return light is then turned into linearly polarized light, which is rotated by 90 degrees with respect to the polarization direction of the incoming path, by transmission through the quarter wavelength plate 103. Thereafter, the return light is turned into convergent light by the collimator lens 104, and then reflected on the beam splitter 102. The return light reflected on the beam splitter 102 is incident into the photodetector 108 through the multi lens 107, and detected by the photodetector 108.
In the case where information recording/reproducing is performed by collecting light on the information recording surface of the optical information recording medium 130, using the optical head device 120, there are generated mainly two kinds of spherical aberrations resulting from a thickness error of the light transmissive layer 131 of the optical information recording medium 130, i.e., aberration due to defocus, and spherical aberration. The aberration due to defocus is corrected by focus servo control. Specifically, the aberration due to defocus is corrected by moving the objective lens 106 in the optical axis direction by the two-axis actuator 109, based on a focus servo signal from the photodetector 108, whereby the laser light is focused on the information recording surface.
On the other hand, the spherical aberration is corrected by turning laser light to be incident into the objective lens 106 into divergent light or convergent light, and generating a spherical aberration having a polarity opposite to the polarity of a spherical aberration which is generated depending on the thickness of the light transmissive layer 131. Specifically, laser light to be incident into the objective lens 106 is turned into divergent light or convergent light by moving the collimator lens 104 in forward or backward direction along the optical axis direction by the collimator lens actuator 109 to generate a spherical aberration having an opposite polarity by the objective lens 106, whereby a spherical aberration resulting from a thickness error of the light transmissive layer 131 is cancelled. In this way, in the conventional optical head device 120, a spherical aberration is cancelled when laser light is focused on the information recording surface through the objective lens 106.
There is proposed an idea of forming a multilayer structure of information recording surfaces to further increase the capacity of an optical information recording medium. In the case where a multilayer structure of information recording surfaces is formed, information recording and/or reproducing is performed with respect to plural information recording surfaces. However, since the distance from the objective lens 106 differs with respect to the information recording surfaces, a spherical aberration is generated on an information recording surface displaced from an optical transmissive layer thickness of an optical information recording medium, which minimizes a residual aberration at the time of parallel light incidence, in other words, an optimal substrate thickness of the objective lens 106, depending on a light transmissive layer thickness Δd from an optimal position which minimizes a residual aberration to an intended information recording surface. A third-order spherical aberration W is expressed by the following formula (1), where Δd is a light transmissive layer thickness from an optimal position which minimizes a residual aberration to an intended information recording surface, n is a refractive index of a light transmissive layer, and NA is a numerical aperture of an objective lens (see e.g. non-patent literature 1).
                    W        =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                                          ⁢          d                                    (        1        )            
As described in the above formula (1), the third-order spherical aberration W is increased in proportion to the light transmissive layer thickness Δd from an optimal position which minimizes a residual aberration to an intended information recording surface. In other words, in the case where a multilayer structure is formed of information recording surfaces, as the light transmissive layer thickness is greatly changed, a spherical aberration amount to be corrected is also increased. As a result, in the conventional optical head device 120, the moving range of the collimator lens 104 is exceedingly increased. As the moving range of the collimator lens 104 is increased, and a degree of convergence/divergence of laser light to be incident into the objective lens 106 is increased, a fifth-order spherical aberration is generated in the objective lens 106.
For instance, let it be assumed that the wavelength λ of a light source is 405 nm, the NA of an objective lens is 0.85, the focal length of the objective lens is 1.3 mm, the focal length of a collimator lens is 19.0 mm, and the optimal substrate thickness of the objective lens (corresponding to a light transmissive layer thickness of an optical information recording medium which minimizes a residual aberration at the time of parallel light incidence) is 87.5 μm. FIG. 9 shows the amount of fifth-order spherical aberration which is generated at the time of correcting a third-order spherical aberration resulting from a change in the light transmissive layer thickness by moving the collimator lens in the above arrangement. As shown in FIG. 9, in an optical information recording medium whose light transmissive layer thickness is changed from 25 μm to 100 μm, even if the third-order spherical aberration is corrected, the amount of fifth-order spherical aberration as a residual aberration becomes maximally 40 mλ. Thus, an influence of a fifth-order spherical aberration in information recording and/or reproducing is not negligible.
Patent literature 1: JP Hei 11-259906A
Non-patent literature 1: “Optical Disk Technology” by Noboru Murayama, Kazusa Yamada, Hiroshi Koide, and Makoto Kunikane, published by Radio Technology Publications Inc., January 1989, p. 60